The present invention relates to radar imaging, in general, and more particularly, to a method of correcting for focus errors or higher order errors, or various combinations thereof, in the final image effected by the radar processing.
Radar imaging may be performed, for example, by synthetic aperture radar (SAR) processing which produces the equivalent effect of an extremely long antenna to improve the azimuth resolution (angular resolution) for high-resolution imaging, like ground mapping, for example. The extremely long antenna may be synthesized as illustrated in FIG. 1 using the forward motion of an aircraft 12' along a flight path 10' to carry a side-looking antenna 14', for example, to flight positions S.sub.1 through S.sub.N which may be treated as individual antenna elements of a linear antenna array. Since these aircraft positions S.sub.1 -S.sub.N or synthesized individual antenna elements do not exist simultaneously, the radar return signals, which contain image phase information, are collected at each position S.sub.n and retained in a memory to be later combined in proper phase to collectively synthesize the desired effect of a narrow-beamed antenna.
An exemplary two-stage radar processor suitable for embodying radar imaging techniques is described in the U.S. Pat. No. 4,034,370 entitled "Second Order Motion Compensator For High Resolution Radar" issued July 5, 1977 to James H. Mims, and assigned to the same assignee as the instant application. Typically, as shown in FIG. 1, a radar may transmit a group of pulsed beams 16' of a predetermined number with a given pulse repetition frequency (PRF) at each aircraft position S.sub.n. Each group of beams may be directed to a common swath or target field 18' on the ground, for ground mapping purposes. The common ground swath 18' may be arranged into a predetermined number of isorange contours 20' which divide the ground swath 18' into a number of range cells R.sub.1 through R.sub.J. Accordingly, for each aircraft position S.sub.n, the radar return signals are collected and processed corresponding to their range cells in a first stage processor of the radar. More specifically, the first stage processor conventionally comprises a fast Fourier transform (FFT) processor which includes a given number of doppler filter banks, say on the order of 16 or 32, for example. Thus, for each range cell R.sub.j, the first stage processor may divide the ground swath 18' into subbeams A.sub.1 through A.sub.k in the azimuth direction commensurate in number to the number of doppler filter banks of the first stage processor, whereby the detected targets in the radar beam 16 may be identified by the range cell R.sub.j and doppler filter bank (subbeam) A.sub.k in which they fall.
Each range-doppler cell (R.sub.j A.sub.k) computation of the first stage processor will result in a complex signal representation of the target corresponding to that cell. The position of a detected target within a subbeam A.sub.k may be measured as a function of its displacement in the azimuth dimension from the equivalent center of its containment filter. The function may be one of a sine wave whose frequency depends on the target displacement from the filter's center frequency. The first stage signal processor may arrange it so that the center frequency of each filter is at zero doppler. Generally, a target which is detected at random doesn't necessarily fall on a filter center line, so it may have some doppler frequency shift associated therewith. A history of complex signals for each target corresponding to the aircraft positions S.sub.1 through S.sub.N is collected and retained in memory for further processing in a second stage processor. The overall observation time during which target signal history is collected is generally referred to as a radar look.
The second stage processor of the SAR is operative to increase the resolution of each azimuth subbeam of the radar ground map 18'. For example, the ground map image after the first processor may only be divided into say 16 subbeams along the azimuth direction corresponding to the number of filter banks utilized in the FFT processing of the radar echo beam data. The radar may collect on the order of 64 time samples corresponding to the aircraft positions S.sub.1 through S.sub.N for each range-subbeam cell R.sub.j A.sub.k of the radar image map. Accordingly, the second processor operates on the time signal history (i.e. 64 time samples) of each range-subbeam cell R.sub.j A.sub.k individually to increase the resolution thereof by as much as 32 fold for the present example. Thus, at the output of the second processor, the map image cell resolution along the azimuth direction may be increased to 32.times.16 or 512 final subbeams for each radar look.
There are of course, certain requirements to be maintained during the radar signal processing of each radar look. One of these requirements is that the aircraft fly at a near constant velocity in a straight line or at least within a narrowly defined corridor. Since this is not always possible, motion compensation systems are provided, such as the one described in the aforementioned reference U.S. Pat. No. 4,034,370, for example, to correct for the aircraft's deviations from that required. For high resolution SAR's, severe motion compensation requirements are imposed and some form of manual or automatic focus adjustment of the radar image generated is desirable. For example, at x-band, the on-board navigational system of one known SAR must measure flight path deviations as small as a tenth of an inch over a flight path of several hundred feet if the signal processor thereof is to produce on infocus image "open loop". Moreover, in these high quality SAR systems, it is also desirable to maintain very low sidelobe levels, say on the order of -40 dB, for example. This is not an easy task because unmeasured mechanical vibrations of only 0.002 inches can produce side lobes on this order. In fact, there are many sources of errors, such as propagation effects and phase and amplitude errors of RF components which may range with the environment, for example, which are also not measureable and may contribute to sidelobe levels. Errors of this variety are commonly referred to as higher order errors.
Present SAR's such as the one described in the aforementioned reference U.S. Pat. No. 4,034,370, for example, generally include a technique for focusing the radar generated image either automatically or manually. One known automatic focusing technique is based on a statistical average of phase signal time histories from selected point-like targets. The phase measurements of this technique are made at an intermediate point in the processing, generally after the first stage, where the correlation gain is high enough to provide adequate signal-to-noise ratio (SNR) for the phase calculations while having sufficient resolution to separate isolated targets from one another. The method consists of a least square error (LSE) fit to a constant frequency i.e. a quadratic phase fit, over the field of selected target signals. While this method has performed adequately in fully automatic operation with real data, it is limited to only measuring the quadratic component of phase error. Extension of this method to include compensation of the higher order errors such as periodic and random (but correlated) errors is unlikely to provide acceptable results. One reason is that the calculation of target phase signal history is presently a non-linear operation (i.e. non-coherent averaging of the complex signals or vectors) which may render 2.pi. ambiguities in the time/signal history. This leads to spurious signal components or glitches which if applied as a correction signal may generate additional errors. Another reason for unacceptability is that amplitude errors are not compensatable by the present focus method.
In summary, the present focus error compensation method involves highly non-linear operations, requires high threshold signal-to-noise ratio levels, and exhibits phase glitches due to the 2.pi. phase ambiguities generated in the calculation of the target phase histories. Accordingly, it would be desirable to avoid most of these problems in a focus error compensation method of a radar, but even more so, the overall compensation provisions of the radar signal processing should be extendable to correct for higher order errors which inevitably arise and cause problems in radar imaging. With the present compensation method, this is not adequately realizable.